Rapid increases in demand for food and energy as a result of population growth and economic development is placing ever increasing demands on limited water resources in South Asia, and climate change is expected further complicate water resource management. In spite of important reductions in poverty levels in recent decades the region is still home to a very large number of poor whose quality of life is directly affected by the availability and quality of water and water services. A significant fraction of the water resources of the region and a significant fraction of the poor are associated with major Himalayan transboundary rivers, and given growing water demand it is likely that the already significant sensitivity around water cooperation amongst co-riparians will increase. Understanding the risks and opportunities for transboundary cooperation in the river systems in South Asia is thus important for guiding sustainable transboundary basin management in the region. This study refines a novel method for a rapid assessment of these cooperation risks and opportunities and applies it to the Brahmaputra, Ganges and Indus river basins to test its utility. The method employs a fuzzy synthetic evaluation technique that combines fuzzy logic and an analytical hierarchy process to assess cooperation risk and opportunity in terms of a Risk-Opportunity Index (ROI). The ROI is a function of four composite development variables and three hegemony variables that indicate the various pressures on the basin water resource and the different control strategies riparians could adopt given existing power asymmetries. In the absence of a clear rationale for differential weighting, equal weights were assigned to all seven variables for this application. A “defuzzification” scoring method is used to define compromising, risk-averse and risk-taking variants of ROI for riparian pairs within each basin. Overall, the results for the compromising ROI suggest that the opportunities for bilateral cooperation are highest (and risks the lowest) in the Brahmaputra Basin and the opportunities are lowest (and the risks highest) in the Indus Basin. This overall assessment is consistent with current common perception. Within the basins the compromising ROI values suggest a few instances of high risk and/or low opportunity, as well as an approximately equal number of instances of medium risk/opportunity and low risk/high opportunity. The study demonstrates that the fuzzy synthetic evaluation technique has utility for rapidly identifying potential opportunities for riparian cooperation in transboundary basins, in order to guide dialogue processes and more detailed analyzes. The study also however, reveals some aspects of the method where further refinement would likely yield more reliable assessments of cooperation risks and opportunities. Specifically, further refinements could consider the relative geographic position of co-riparians within a basin, and the relative resource access of different riparians. The method only considers bilateral riparian interactions and not more complex multi-lateral interactions. The results of study may contribute to various ongoing regional and basin dialogues on water cooperation in South Asia. © 2017, Springer Science+Business Media Dordrecht.